In recent years, problems have been raised on global environment such as global warming and the like caused by exhaust gas and the like exhausted from transportation vehicles to begin with automobiles, and, as one of the countermeasures, improvement of the fuel economy by weight reduction of a vehicle such as an automobile has been studied. On the other hand, only with the weight reduction of a vehicle, there is a fear that the possibility of a collision accident of an automobile and the like to become a big accident increases. Therefore, in addition to weight reduction, improvement of safety has also been studied. Due to such reasons, application of an aluminum alloy light in weight and excellent in energy absorption to a part of a vehicular body of a transportation vehicle to begin with an automobile has been studied, and such the aluminum alloy has reached also to practical use in recent years.
However, in order to apply an aluminum alloy to a transportation vehicle to begin with an automobile, it is necessary to execute structural design utilizing the properties of an aluminum alloy. Therefore, with the exception of special vehicles of completely made of an aluminum alloy, an aluminum alloy material is partially incorporated into a steel material that constitutes a structural body of a vehicular body. Accordingly, a dissimilar metals bonding technique of a steel material and an aluminum alloy material becomes indispensable.
However, when dissimilar metals bonding of a steel material and an aluminum alloy material is to be executed only by welding, an intermetallic compound layer (or a reaction layer) of Fe and Al highly hard and highly brittle is formed in the bonding interface of the steel material and the aluminum alloy material. That is, such a problem occurs that, even when the steel material and the aluminum alloy material appear to be bonded to each other, sufficient bonding strength cannot be secured in the joint due to the highly brittle intermetallic compound layer (or the reaction layer) of Fe and Al formed in the bonding interface.
When welding is employed for the bonding technique of a steel material and an aluminum alloy, such a problem as described above occurs. Therefore, for bonding of a steel material and an aluminum alloy material, mechanical bonding techniques using fixtures such as self pierce rivets, bolts and the like and bonding techniques using adhesives have been employed. However, these bonding techniques have the problems of complicated bonding work and increase of bonding cost, and have not been applied widely.
Currently, for bonding of a steel material and the like which constitute a transportation vehicle to begin with an automobile, welding techniques such as linear bonding such as TIG welding and the like and spot bonding and the like are generally used. When these welding techniques can be applied to the dissimilar metals bonding technique of a steel material and an aluminum alloy, a vehicle using an aluminum alloy can be manufactured in a line and process same to those for the steel material in a factory, and efficiency of the bonding work can be further enhanced.
Among these welding techniques, TIG welding is a technique employed most widely for bonding of steel materials to each other and the like. However, as described above, there is a fear of a problem of formation of an intermetallic compound layer of Fe and Al in dissimilar metals bonding of a steel material and an aluminum alloy material, and therefore the fact is that not only TIG welding has not been put in practical use but also it has not been proposed.
Under such situation, in Patent Literature 1, a technique on dissimilar metals bonding for a steel material and an aluminum alloy material by TIG welding has been proposed. However, in this proposal, TIG welding has been just cited as an example of butt welding and lap welding. More specifically, in this proposal, the welding means is selected from a group consisting of a laser, electron beam, plasma arc, TIG, MIG, and CO2 arc. In this proposal, it is only mentioned that a welding means can be TIG welding.
In Patent Literature 2, a dissimilar metals bonding method has been proposed in which a steel material and an aluminum alloy material are welded so as to be lapped each other with the steel material being laid in the upper side and the aluminum alloy material in the lower side relative to the welding operation direction. In this method, it has been proposed to execute welding along the welding line in a state the position of the welding surface of the aluminum alloy material is projected to the upper side of the position of the welding surface of the steel material relative to the welding operation direction. It has been described that, according to this method, the wettability of the welding surface of the steel material against the aluminum molten metal is improved, removal of an oxide film on the surface (welding surface) of the steel material is promoted, and excellent bonding can be achieved.
Also, in this Patent Literature 2, TIG welding of a form using a flux-cored wire (FCW) that is obtained by filling flux inside an outer sheath of an aluminum material is also intended. In the meantime, a number of compositions of FCW used for such dissimilar metals bonding have been proposed from the past in Patent Literature 3 and the like. However, even in these Patent Literatures 2, 3, a concrete dissimilar metals bonding method of an aluminum alloy material and a steel material using TIG welding has not been disclosed.
Therefore, the present inventors newly executed an actual dissimilar metals bonding test for bonding an aluminum alloy material and a steel material by TIG welding, and confirmed the problems occurring then. More specifically, as shown in FIG. 10, first, one end of an aluminum alloy material 11 was overlaid on top of a steel material 12 with the aluminum alloy material 11 being placed on the upper side. Also, while feeding a flux-cored wire (FCW) 7 to a step section 13 formed by an end 11a of the aluminum alloy material 11 and the surface of the steel material 12, the dissimilar metals bonding test was executed for fillet welding of the aluminum alloy material 11 and the steel material 12 using a tungsten electrode 30. Further, TIG welding was executed by AC TIG welding that was generally used for welding of aluminum alloy materials to each other.
As a result of this dissimilar metals bonding test, although the spatter scarcely occurred in welding which was excellent, the molten aluminum alloy material 11 and the FCW 7 that was molten similarly became a bonding (bead) of being present separately as beads 4a, 4b on the surface of the steel material 12.
Such bonding state of separation into the beads 4a, 4b occurred even when adjustment of largely changing the welding condition of TIG welding such as increasing the power (heat input), reducing the welding speed and the like was executed, and could not be entirely prevented. In such bonding state of the separate bead, the bonding strength of the joint (dissimilar metals bonding joint) cannot be increased.
Also, this separation of the bead is liable to occur particularly under high pressure of an arc and under welding at a high speed in TIG welding of steel materials to each other, and is known as a humping phenomenon in which the shape of a molten pool becomes unstable, the molten pool vibrates, and severe unevenness occurs in the bead.
As a means for preventing occurrence of this separation of the bead, a method using a magnetic field and a technique exercising ingenuity in the shape of the tip section of the tungsten electrode have been proposed although they are only the technique on TIG welding of steel materials to each other. A method using a magnetic field has been proposed for example in Patent Literature 4. In the Patent Literature 4, an electromagnetic force generated by the electromagnetic interaction of a constant magnetic field applied to the periphery of a welding arc and the arc current flowing in the welding arc is applied to the welding arc to deflect the radiation shape of the welding arc to the welding line direction of the section to be bonded, and thereby two sections to be bonded are bonded continuously.
Although such the method using a magnetic field as disclosed in the Patent Literature 4 is a technique effective in preventing occurrence of separation of the bead without any question, a magnet device becomes necessary in addition. Also, according to this method, because an electromagnetic force generated by the magnet device is applied to the welding arc, control thereof is required, and it is not possible for an existing welding facility to cope with simply. It is a problem practically because the cost increase is involved.
On the other hand, techniques exercising ingenuity in the shape of the tip section of a tungsten electrode have been proposed in Patent Literature 5 and Patent Literature 6. The shape of the tip section of a general tungsten electrode is a circular conical shape similar to that of the tip of a pencil. However, in the Patent Literature 5, proposed is an electrode having a shape in which a point on the surface of an electrode for TIG welding having a ridge line at the tip section is made the distal end, and the angle formed by surfaces forming the ridge line is made 40-100 degrees. In the Patent Literature 5, the arc point is generated along a portion of the ridge line, the arc is prevented from climbing up to a portion of a surface in the vicinity of the tip of the electrode, the arc is prevented from widening in the width direction of the bead to concentrate the arc, and TIG welding with big current and high speed is tried to be enabled.
In the Patent Literature 6, it is proposed, in a TIG welding apparatus for bonding welding of steel materials having narrow beveling to each other, to direct a tungsten electrode toward a welding line from above an aluminum alloy material, to make the tip of the tungsten electrode of an eccentric shape diagonally cut at an angle of 30°-40°, and to melt the wire while rotating the tungsten electrode inside the narrow beveling.
Even when the electrode improvement technique in TIG welding of the steel materials to each other described in the Patent Literature 5 is applied to TIG welding of dissimilar metals of an aluminum alloy material and a steel material using an FCW, occurrence of separation of the bead described above cannot be prevented. The reason is considered that TIG welding of dissimilar metals of the aluminum alloy material and the steel material using the FCW is welding of dissimilar materials to each other, and therefore the welding mechanism, the generation mechanism of separation of the bead and the like thereof are different from those of TIG welding of steel materials to each other.
Also, the technique described in the Patent Literature 6 is a technique on butt welding of thick plates, and cannot be applied to lap fillet welding of thin sheets as it is.